1. Field of the Invention
The present invention relates to a nonlinear optical element and more particularly to the one which can be used as devices in light-(electromagnetic wave-) related fields, that is, optical and optoelectronic fields. Also, the present invention relates to uses of such a nonlinear optical element.
2. Description of the Prior Art
Optical information processing or optical communication at higher speeds and higher densities must necessarily require the use of nonlinear optical elements, and research and development of materials having higher nonlinear optical susceptibilities are now widely under way.
Various methods have been used for controlling the form of such materials with view to fabricating devices therefrom, which methods include (1) crystal growth methods in which large single crystals, single thin film crystals, etc. are formed; and (2) thin film formation methods such as Langmuir-Brodget (BL) method, vacuum vapor deposition method, spincoating method, and the like.
While the aforementioned methods (1) and (2) are currently most commonly used, they remain to merely change the form of the materials to ones suitable for the purposes for which they are used; the performance of the devices fabricated depends on the nonlinear optical characteristics intrinsic to the materials used. Therefore, it is practically very difficult to obtain nonlinear optical elements with higher speeds and higher densities because development of optical materials having higher nonlinear susceptibilities seems rather stagnant at present.
One approach for solving the aforementioned problem is to utilize spherical microcavities made of conventional nonlinear optical materials, exhibiting photoconfinement effect, with view to inducing nonlinear optical phenomena at low thresholds.
Studies on the confinement of light with a certain wavelength into a microsphere with a diameter of about several tens microns (.mu.) and lasing using such a system have been made focusing on liquid droplets. Opt. Lett., 9, 499 (1984); Opt. Lett., 11, 614 (1986); and Opt. Lett., 15, 980 (1990) describe dependence of the confinement of light into liquid droplet microspheres containing a dye and lasing therefrom on the size of the microspheres as well as thresholds therefor. Because of their free adjustability for diameter or size to any desired values and of their ease of changing the concentration of the dye, liquid droplets are useful as a sample for fundamental measurements. However, they are unsuitable for fabricating devices therewith because they lack a permanent stability.
On the other hand, some studies have been made on solid microspheres.
(1) In Phys. Rev. Lett., 44, 475 (1980) there is a report on the study of confinement of light into a polystyrene microsphere containing a fluorescent dye. However, no mention is made therein of nonlinear optical phenomena including lasing. Measurements were made of a population of the microspheres suspended and buoyant in a liquid medium. Therefore, this system, like the liquid droplets, is unsuitable for application to fabricating optical devices.
(2) Phys. Lett. A, 137, 393 (1989) describes the efficiency of confinement of light into a quartz microsphere, calculation of the threshold value of optical bistability and some experimental results. In this study, the quartz microsphere is set on a small quartz stem for the convenience of measurements to avoid influences of heat thereon but no idea on the fabrication of optical devices is found. Furthermore, all optical bistable phenomenon has not been observed on quartz which constitutes the aforementioned optical system singly, resulting in that no effective nonlinear optical element has yet been established using quartz alone.
As described above, photoconfinement effects with microspheres have been studied mainly by conducting measurements on a population of buoyant microspheres. As far as is known, it has not been intended to fix or immobilize microspheres for fabricating optical devices excepting for merely conducting fundamental measurements of the optical characteristics of a microsphere itself.